Only 10 percent of the trillions of cells that make up your body are yours. The rest are bacteria, tiny single-celled microbes that dwell in and on almost every part of you. Most of these bacteria are beneficial, synthesizing vitamins and helping digest your food. As much as 30 percent of the calories you get from some foods comes from the actions of bacteria in your gut.

But some bacteria are deadly, as recent headlines about deaths from MRSA (methicillin-resistant Staphylococcus aureus) attest. Fatalities caused by this bacteria are increasing. In 2005, MRSA claimed the lives of 19,000 people in the United States, more than died of AIDS. Furthermore, other ailments associated with the body's immune system -- hay fever and irritable bowel diseases, for instance -- are also on the rise. In her comprehensively documented and well-crafted book, Jessica Snyder Sachs explains what's behind this bacterial onslaught. The two most likely sources of the increase in bacterial infections seem, in many ways, the most unlikely: improved public sanitation and the widespread use of antibiotics.

The war on germs (the layman's word for infectious bacteria and other microbes) began in earnest in the middle of the 19th century. One of the leaders was Florence Nightingale, who championed the "cleanliness is next to godliness" approach to public health. Nightingale and others had a profound impact on sanitary conditions in Europe and America. The improved sanitation they advocated largely stopped the cycle of waterborne epidemics that began with the crowding of civilization. This revolution in public health nearly doubled average life spans in the United States, from 38 years in 1850 to 66 in 1950.

But the cleanliness revolution had a downside. "Throughout the developed world," Sachs writes, "allergies, asthma and other types of inflammatory disorders have gone from virtually unknown to commonplace in modern times." The reason for this increase was unknown until a Scottish epidemiologist began studying the health and habits of thousands of Britons. "Over the past century," he concluded, "declining family size ... and higher standards of personal cleanliness have reduced the opportunity for cross infection in young families." The upsurge in allergies and asthma, he said, was due to the decrease in the infections of childhood, which produced a weakened immune system in adults. Conversely, people who suffered a lot of runny noses as children were less likely to have them when they grew up.

As the revolution in public sanitation was ending in the 1950s, a new and equally important revolution was beginning: the widespread adoption of antibiotics. The first antibiotics were sulfa drugs. Then came penicillin. Others soon followed. These drugs were spectacularly successful in fighting bacterial diseases such as strep throat, scarlet fever and staph infections. But by 1955, new strains of bacteria had appeared that resisted treatment. Particularly noxious was a strain of Staphylococcus aureus, the so-called superbug behind today's outbreak of MRSA. The 1950s strain, Sachs writes, "shrugged off not only penicillin but every antibiotic on the pharmacist's shelf."

Scientists countered by developing methicillin, which worked well until the mid-1980s when methicillin-resistant strains developed. Hospitals then began treating staph-infected patients with vancomycin. To the surprise of no one, a vancomycin-resistant staph infection was reported in 2002.

To combat resistant strains, scientists developed ever more deadly drugs that indiscriminately attack the body's bacteria. These "big guns" were especially useful for doctors in critical-care situations when there was no time to run tests to determine which germ was causing the illness. Heavily promoted by big pharma, broad-spectrum antibiotics quickly became physicians' first line of defense against infections.

"But all this convenience had a dark side," Sachs writes. "The scattergun attack of a broad-spectrum antibiotic razes not only the disease-causing organism that is its intended target but also the body's ... protective and otherwise beneficial microflora." When powerful antibiotics eliminate beneficial bacteria, it allows less friendly strains to flourish. One study showed that the longer hospital patients took antibiotics, the greater their risk of acquiring new infections.

What can be done to solve the problems created by improved public sanitation and overuse of broad-spectrum antibiotics? A return to the good old days of dirty water and no antibiotics is clearly out of the question. Thanks in large part to clean water and antibiotics, a person born in the United States can now expect to live to the ripe old age of 78. No, going backward is not the answer. But the problems of allergies and antibiotic resistance are real, and Sachs trots out some possible solutions.

Some approaches, such as a "dirt vaccine" that aims to stimulate the immune system, have produced mixed results at best. But others, like probiotic techniques (in which a friendly strain of bacteria is used to inhibit the growth of less friendly strains), have promise and some demonstrated successes.

Sachs aims her strongest, and most practical, recommendations at the twin problems caused by overexposure to broad-spectrum antibiotics: drug resistance and the inhibition of beneficial bacteria. Overuse of these antibiotics wipes out all but a few bacteria. The survivors have some form of resistance, and with all the other bacteria gone, the resistant strains flourish. Reducing antibiotic use kills fewer good bacteria, increasing the competition that holds the resistant bugs in check. Such a reduction is possible, Sachs contends. "Unnecessary prescriptions still account for around one-third of all the antibiotics we take." We also stay on antibiotics too long. Recent data indicate that courses shorter than the recommended length are just as effective for many infections.

Sachs also advocates using less disruptive drugs. When presented with an infected patient, many doctors reach for the biggest gun in their arsenal, usually a broad-spectrum antibiotic. This "shoot first, aim later" approach is easier for doctors, but hard on the body's beneficial bacteria. Sachs encourages doctors to use more targeted drugs, antibiotics that just go after the bad guys.

The common-sense remedies offered by Sachs are not new. The problem is getting them adopted. As one doctor put it, "it's just easier to prescribe the broad-spectrum and not have to worry about follow-up." However, Sachs believes that a gentler approach is gaining acceptance in which the treatment of bacterial disease is "less a war on an invisible enemy than a restoration of balance." After all, she points out, this is "and always will be, a bacterial world."